1. Field of Invention
The present invention relates to a process for preparing cyclohexanol and cyclohexanone by cyclohexane oxidation, and more particularly to a process for preparing cyclohexanol and cyclohexanone by cyclohexane oxidation comprising a three-step decomposition process for decomposing the cyclohexyl hydrogen peroxide.
2. Description of Related Arts
The conventional process for the preparation of cyclohexanol and cyclohexanone comprises: non-catalyticly oxidizing cyclohexane with molecular oxygen to obtain an oxidized mixture containing cyclohexyl hydroperoxide (CHHP) as a main product; decomposing the CHHP to obtain cyclohexanol and cyclohexanone; and rectifying to obtain products of the cyclohexanol and the cyclohexanone. Internationally, the art of decomposing the CHHP to obtain the cyclohexanol and the cyclohexanone comprises two manners: the homogeneous catalytic decomposition by bis(tert-butyl) chromate, disclosed by French Rhodia Company; and, the non-homogeneous catalytic decomposition by cobalt acetate in the alkaline aqueous solution of sodium hydroxide, disclosed by Dutch DSM. Both of the two manners are improved low-temperature decomposition techniques based on the conventional homogeneous catalytic oxidation process and the conventional saponification decomposition process by sodium hydroxide aqueous solution developed in the 1950s to 1970s.
Though having a molar yield up to 94% in the decomposition process, the homogeneous catalytic decomposition of CHHP by the bis(tert-butyl)chromate has two serious defects. Firstly, during decomposing, the scale formation, mainly the chromium adipate, blocks equipments and pipelines. Disclosed by Rhodia, the phosphoric acid octyl ester is used as the scale inhibitor, which fails to completely solve the scale formation. The continuous production cycle only lasts for four months; washing and descaling after stalling the production device are executed three times per year. Secondly, the conversion rate is low, wherein the molar conversion rate is only around 92%; and around 5% of the CHHP still remains in the decomposed materials. The remaining CHHP is decomposed under the conditions of a high concentration of cyclohexanol and cyclohexanone, high acidity and a high temperature inside the cyclohexane recycling columns and the cyclohexanol and cyclohexanone product columns, so as to mainly produce acid compounds, like adipic acid, and ester compounds, mainly caprolactone; to speed up the condensation reaction of free radicals of the cyclohexanol and the cyclohexanone, and the esterification reaction of cyclohexanol; and to generate the high-boiling-point substances, and reduce the yield. Conventionally, the total molar yield of the domestic and foreign devices thereof is only around 80%.
The non-homogeneous catalytic decomposition by cobalt acetate in the alkaline aqueous solution of sodium hydroxide also has three defects. Firstly, the decomposition causes the big secondary reaction, and has a low molar yield of only 84%. Secondly, it is difficult to completely separate the cyclohexane oil phase containing cyclohexanol and cyclohexanone from the alkaline aqueous phase containing the alkaline waste. The oil phase always contains a certain amount of the waste alkaline aqueous phase, in such a manner that the scales of the waste alkaline are always formed in the rectification columns subsequently, which blocks the rectification columns and the reboilers thereof, and results in the continuous production cycle of only six months. Thirdly, the decomposition has a high alkali consumption and a large discharge capacity of alkali waste. A concentration of OH− ion in the alkali waste must be controlled around 1 mol/L. If the concentration of OH— ion is over 1 mol/L, difficulty of combustion of alkaline waste is increased; if below 1 mol/L, decomposition conversion rate is low, and part of the cyclohexyl hydrogen peroxide is decomposed continuously in an cyclohexane recycling column and a cyclohexane and cyclohexanone product column in the subsequent process, and caprolactone and organic acids are mainly generated, which decreases the total yield of the device and influences quality of cyclohexanone products. Currently, the conventional industrial devices adopting the process have the total molar yield of only about 80%.
Conventionally, the worldwide companies respectively adopt one of the above two manners to accomplish decomposing the CHHP at one step. The Chinese patents ZL9411039.9 and ZL98112730.4, filed by the inventor of this application, disclose the two-step decomposition art. At the first step thereof, the alkalinity is lowered; the recycling amount of the alkaline aqueous phase is increased; the static mixer is used. Industrial application results indicate that, the total molar yield of the device thereof really increases, but the separation of the cyclohexane oil phase from the waste alkaline aqueous phase becomes more difficult. The several sets of industrial production devices of the whole two-step decomposition art have a molar total yield of around 82%.